When functioning properly, the human heart maintains its own intrinsic rhythm based on physiologically-generated electrical impulses. It is capable of pumping adequate blood throughout the body's circulatory system. Each complete cycle of drawing blood into the heart and expelling it is referred to as a cardiac cycle.
However, some people have abnormal cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. Arrhythmias can occur in the upper chambers of the heart—the atria, or the lower chambers of the heart—the ventricles. However, ventricular arrhythmias present the most serious health risk as they can lead to rapid death from the lack of circulation. Arrhythmias can be subdivided further into specific conditions of the heart that represent vastly different manifestations of abnormal cardiac rhythm. These conditions are bradycardia, or a slow heartbeat, and tachycardia, or a fast heart beat.
One mode of treating a cardiac arrhythmia uses an implantable medical device. Such implantable medical devices include pacemakers, also referred to as pacers, and defibrillators. The traditional use of a pacemaker is to treat a person with bradycardia. In other words, pacemakers help speed up the cardiac cycle of a person whose heart beats too slowly. Pacers accomplish this by delivering timed sequences of low energy electrical stimuli, called pace pulses, to the heart. Such stimuli are delivered via an intravascular lead wire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart.
In comparison to a pacemaker, an implanted defibrillator applies a much stronger electrical stimulus to the heart. This is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The shock changes ventricular fibrillation to an organized ventricular rhythm or changes a very rapid and ineffective cardiac rhythm to a slower, more effective rhythm. Defibrillators help treat cardiac disorders that include ventricular fibrillation, ventricular tachycardia, atrial fibrillation, and atrial flutter. These inefficient or too rapid heartbeats reduce the pumping efficiency of the heart and thereby diminish blood circulation. The countershock delivered by the defibrillator interrupts the tachyarrhythmia, allowing the heart to re-establish a normal rhythm for the efficient pumping of blood.
Another mode of treating a cardiac arrhythmia uses drug therapy. Drugs are often effective at restoring normal heart rhythms. Modern implantable medical devices can be configured to release drugs through specialized leads or a pumping device. Steroids are commonly administered in this manner to suppress inflammation of the heart wall. However, with continuing advances in pharmaceutical research, powerful anti-arrhythmic drugs may also be administered through an implantable medical device.
An implantable medical device also can be configured to include an accelerometer. A tiny crystal sensor inside the device detects body movement and signals the device to adjust pacing of the heart up or down according to the wearer's activity. This technology has been further refined so that modern implantable medical devices can mimic the heart's natural rhythm even more closely by adjusting the rhythm according to a person's activity level. Modern implantable medical devices also can separately sense and coordinate the contractility of both the upper (atria) and lower (ventricles) chambers of the heart and serve as dual pacer/defibrillators, drug delivery devices, and as a component of a comprehensive patient management system for predictive management of patients with chronic disease.
Modern implantable medical devices are becoming smaller (½ the size) and smarter than earlier devices and can last much longer. With the recent introduction of “mode switching,” modern devices can now, for example, recognize an abnormally fast heart rate in the upper chamber of the heart and react by automatically changing the therapy the device delivers. This feature allows the device to deliver the most appropriate therapy. Modern implantable medical devices also can collect information and store it until the next clinic visit. Some devices also make follow-up easier by storing patient data directly into the memory of the device (such as name, diagnosis, doctor).
However, by tasking the implantable medical device to do more, the demands on the device's power supply, typically a lithium-iodine battery, increase, such that the device may need to be replaced more often. In order for an implantable medical device to serve multiple functions without having to be frequently replaced, the battery life, and hence, the useful life of the device, must be extended. This can be accomplished by duty cycling the device's major subsystems. In other words, the device is on and using power only during specific periods to help conserve battery power.
Thus, for these and other reasons, there is a need for an implantable medical device that can serve multiple therapeutic purposes for many years without having to replace the device on more than a few occasions, if ever, during the patient's lifetime.